EL devices, which may be generally classified as organic or inorganic, are well known in the graphic display and imaging art. Among the benefits of organic EL devices, such as organic light-emitting diodes, are high visibility due to self-emission, as well as superior impact resistance, and ease of handling of the solid state devices. OLEDs may have practical application for television and graphic displays, as well as in digital printing applications.
An OLED is typically a laminate formed on a substrate such as soda-lime glass. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The semiconductor layers may be hole-injecting and electron-injecting layers. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. The light-emitting layer may consist of multiple sublayers.
When a potential difference is applied across the cathode and anode, electrons from the electron-injecting layer, and holes from the hole-injecting layer are injected into the light-emitting layer. They recombine, emitting light.
In a typical matrix-addressed OLED display, numerous OLEDs are formed on a single substrate and arranged in groups in a regular grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time.
OLEDs have a number of beneficial characteristics. These include a low activation voltage (about 5 volts), fast response when formed with a thin light-emitting layer, and high brightness in proportion to the injected electric current. By changing the kinds of organic solids making up the light-emitting layer, many different colors of light may be emitted, ranging from visible blue, to green, yellow, and red. OLEDs are currently the subject of aggressive investigative efforts.
An OLED may be designed to be viewed either from the "top"--the face opposite the foundational substrate--or from the "bottom", i.e., through the substrate, from the face opposite the light emitting layer. Whether the OLED is designed to emit light through the top or the bottom, the respective structure between the viewer and the light emitting material needs to be sufficiently transparent, or at least semi-transparent, to the emitted light. In many applications it is advantageous to employ an OLED display having topside light output. This permits the display to be built on top of a silicon driver chip for active matrix addressing.
It has been a continuing challenge to devise OLED display structures which provide topside light output while minimizing light blockage and thereby permitting a high-resolution display. Transparent conductors deposited on top of organics, at low temperature, tend to be excessively resistive. This adversely affects the response time of the device or peak brightness. Various methods have been tried including transparent conductors (e.g., ITO and IZO) on thin metal.
Accordingly, there is a need for a top electrode structure for an OLED which permits high resolution displays to be fabricated with low line resistance and topside light output. The present invention meets these needs, and provides other benefits as well.